Device for measuring a characteristic of a color cathode ray tube

ABSTRACT

A device for measuring convergence of a color CRT including image pickup means for producing three color image signals with respect to three color test patterns constituting a composite test pattern presented on a viewing screen of a color CRT, change means for changing the image magnification of said image pickup means according to the pitch between color phosphor elements on the viewing screen, and calculation means for calculating respective luminous centers of gravity of the three color test patterns from the three color image signals, and calculating a misconvergence of the color CRT from the calculated luminous centers of gravity and a changed image magnification.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

This invention relates to a device for measuring convergence of threeprimary color electron beams of red, green and blue of a color cathoderay tube.

The color cathode ray tube (referred to as CRT hereinafter) displays acolor image by directing each of the red, green and blue electron beams,controlled by video signal, correspondingly to the red, green and bluephosphor dots on the inner surface of the faceplate. The faceplate iscomposed of numerous red, green and blue phosphor dots deposited in amosaic pattern.

In adjusting a color CRT, its convergence is measured and adjusted sothat a properly colored display is obtained by assuring that the red,green and blue electron beams pass through any mesh of the shadow maskand accurately strike respective color phosphor dots. It is well knownthat convergence measurement devices are conventionally used to adjustconvergence of the color CRT. Japanese Unexamined Patent Publication No.59-747880 discloses a device in which a white composite test pattern formeasurement is presented on the color CRT for convergence measurement;the test pattern is separated into red, green and red patterns usingcolor filters; each separated pattern is then measured using anindustrial television camera of which output is then used to calculate aluminous center of the test pattern of each color; and calculatedrelative positional difference is considered as a misconvergence to becompensated for.

The convergence measurement using a measurement device mentioned aboverequires a longer measuring time because the red, green, blue color testpatterns are required to be separately picked up prior to theconvergence calculation.

The longer measuring time causes the measurement device to operate underundesirable measurement condition changes such as supply voltagevariations and luminance changes of the CRT due to generation ofexternal noise or due to disagreemnet of the scan timing of the CRT andthe scan timing of the image pickup device while the red, green, bluecolor patterns are individually picked up. Consequently, measurementaccuracy may be degraded.

Also, the conventional device needs a fixture to mechanically keep boththe color CRT and the industrial television camera securely in placeduring color filter changing operations. The fixture causes the size ofthe device to be larger.

Minolta Camera Kabushiki Kaisha, an asignee of the present application,has filed Japanese Patent Application No. 62-259088 which discloses aconvergence measurement device using a color image pickup device tosolve the above-mentioned problems. This convergence measurement deviceenables red, green and blue test patterns to be measured together at atime while, providing one shorter measuring time. The device alsoassures easy measurement operation of simply placing the color filterbuilt-in image pickup device against the viewing screen of the CRT. Thetest pattern on the color CRT are produced by red, green, blue phosphordots which are periodically spaced and mosaically arranged on theviewing screen of the CRT. In a color image pickup device provided witha single filter plate having mosaically-arranged three color filters,also, a great number of pixels are periodically spaced or distributed.It should be noted that a single color phosphor dot is picked up by aplurality of three color pixels. Accordingly, the measurement in whichred, green, and blue test patterns are separately processed involveserrors due to positional relation between the arrangement of the colorphosphor dots on the color CRT and the arrangement of color pixels inthe color image pickup device. When the color image pickup device ismanually handled for quick measurement of convergence, differentmeasurements are liable to generate due to the fact that the color imagepickup device is unavoidably placed on different positions on theviewing screen.

Reasons why such an error takes place will be seen from the followingdescription. FIG. 27 shows a cross hatch pattern displayed on the colorCRT. To measure convergence, a horizontal misconvergence is obtained bya vertical line of the cross hatch pattern and a vertical misconvergenceis obtained by a horizontal line of the cross hatch pattern. An areaindicated at A in FIG. 27 is expanded in FIG. 28 to show in detail.Small circles represent regularly arranged phosphor dots. Phosphor dotsgiven letters R, G, and B represent red phosphor dots, green phosphordots, and blue phosphor dots respectively. A first area where bluephosphor dots are glowing is enclosed by two parallel alternate long andshort dash lines. A second area where red phosphor dots are glowing isenclosed by two parallel alternate long and two short dash lines. Athird area where green phosphor dots are glowing is enclosed by twoparallel dashed lines. Pt is a vertical pitch between the phosphor dotsof one color. FIG. 28 shows a part of the viewing screen of anunadjusted color CRT in which red, green, and blue luminance lines arestill misconverged. A horizontal misconvergence is obtained bycalculating respective luminous centers of gravity of misconverged red,green, and blue glowing lines to provide respective positional data ofthe three color test patterns, and by calculating a difference betweenthe positional data. In the same manner as above, a verticalmisconvergence is obtained in an area indicated at B FIG. 27.

FIG. 29 shows an arrangement of pixels of a color image pickup device.It is noted that a CCD (Charge Coupled Device) has a color filteralternately striped with red, green, and blue parts, and that pixels arearranged vertically as well as horizontally in respective uniformpitches. The pixels given letters R, G, and B represent red pixels,green pixels, and blue pixels respectively. Pt' is a horizontal pitchbetween the pixels of one color. Since Pt' is greatly smaller than colorphosphor dot pitch Pt shown in FIG. 28, the plurality of red, green, andblue pixels receive light from the glowing phosphor dot when the colorimage pickup device is picking up the test pattern.

It is now assumed that a horizontal luminous center of gravity of thegreen phosphor dots is being calculated. The plurality of red, green,and blue pixels receive light from one green phosphor dot. However, thegreen pixels receive the light most strongly. Consequently, the imagepickup device gives a color image of green stripes enclosed by a circleas shown in FIG. 30A and FIG. 30B. Each circle represents a glowinggreen phosphor dot and vertical hatched stripes inside the circle are aresultant image by the green pixels.

FIG. 30A and FIG. 30B show that the luminous center of gravity of thegreen phosphor dot image does not agree with the luminous center of thegreen phosphor dot itself. Two different phosphor dot images on twodifferent columns give different results in the luminous center ofgravity, although the green phosphor dot images on the same column agreeto each other in luminous center of gravity. This is because greenstripes (hatched) on the right hand side column and those on the lefthand side column are out of phase as shown in FIG. 30A.

Deviation thus results as shown in FIG. 30A. A line Ms represents theluminous center of gravity of the glowing green phosphor dots themselvesand a line Mm represents the luminous center of gravity of the glowinggreen phosphor dot image on the image pickup device.

If a deviation occurs in the horizontal positional relation between thepixels of the image pickup device and the color phosphor dots of the CRTunder test, the above mentioned Mm varies. Accordingly, it could be seenthat deviation δ between Mm and Ms changes each time the color imagepickup device is moved. FIG. 30B shows that a horizontal movement of theimage pickup device relative to the phosphor dots causes the deviation δto change with respect to that of FIG. 29A.

When convergence is measured from the luminous center of color phosphordot line of each color, a slight movement of the color image pickupdevice relative to the color phosphor dots at each measurement causesconvergence data to vary, degrading measurement accuracy. This is mainlydue to relative changes in positional relation between the arrangementof phosphor dots on the viewing screen of the CRT and the arrangement ofthe pixels in the color image pickup device. Experimentally, it is foundthat a relation of a phosphor dot pitch of 310 μm (Pt) and a pixel pitchof 30 μm (Pt') causes horizontal convergence data to vary in the term of30 μm or more.

Also, changed positioning of the image pickup device varies the positionof calculation area for luminous center of gravity, consequently causingan increased measurement fluctuation.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention is to provide adevice for measuring convergence of a color CRT which makes it possibleto keep measurement variations to a minimum by properly controllingimage magnification of an color image pickup device even when the colorimage pickup device is placed on any position of the viewing screen ofthe CRT.

A device for measuring convergence of a color CRT of the presentinvention comprises image pickup means for producing three color imagesignals with respect to three color test patterns constituting acomposite test pattern presented on a viewing screen of a color CRT,change means for changing the image magnification of the image pickupmeans according to the pitch between color phosphor elements on theviewing screen, and calculation means for calculating respectiveluminous centers of gravity of the three color test patterns from thethree color image signals, and calculating a misconvergence of the colorCRT from the calculated luminous centers of gravity and a changed imagemagnification.

Also, a device for measuring convergence of a color CRT of the presentinvention comprises image pickup means for producing three color imagesignals with respect to three color test patterns constituting acomposite test pattern presented on a viewing screen of a color CRT, afirst setting means for setting a first horizontal sampling area havinga length which is obtained by multiplying a pitch between the samehorizontal color phosphor elements on the viewing screen and an integer,a first calculation means for calculating first respective verticalluminous centers of gravity of the three color test patterns from thethree color image signals from the first horizontal sampling area, asecond setting means for setting a second horizontal sampling area whichis deviated in a horizontal direction by a given amount from the firsthorizontal sampling area, a second calculation means for calculatingsecond respective vertical luminous centers of gravity of the threecolor test patterns from the three color image signals from the secondhorizontal sampling area, and misconvergence calculation means forcalculating a vertical misconvergence of the color CRT from the firstand second vertical luminous centers of gravity calculated by said firstand second calculation means.

Further, a device for measuring convergence of a color CRT of thepresent invention comprises image pickup means for producing three colorimage signals with respect to three color test patterns constituting acomposite test pattern presented on a viewing screen of a color CRT, afirst setting means for setting a first vertical sampling area having alength which is obtained by multiplying a pitch between the samevertical color phosphor elements on the viewing screen and an integer, afirst calculation means for calculating first respective horizontalluminous centers of gravity of the three color test patterns from thethree color image signals from the first vertical sampling area, asecond setting means for setting a second vertical sampling area whichis deviated in a vertical direction by a given amount from the firstvertical sampling area, a second calculation means for calculatingsecond respective horizontal luminous centers of gravity of the threecolor test patterns from the three color image signals from the secondvertical sampling area, and misconvergence calculation means forcalculating a horizontal misconvergence of the color CRT from the firstand second horizontal luminous centers of gravity calculated by saidfirst and second calculation means.

As mentioned above, in a device of the present invention, a compositetest pattern presented on the viewing screen of the CRT is so magnifiedthat the pitch between color phosphor elements on the viewing screen andthe pitch between pixels on the color image pickup means have aspecified proportional relationship to each other, and can be picked upby the image pickup means. The color image pickup means separates thecomposite test pattern into three color test patterns or three colorimage signals. The three color image signals are fed to a video memorywhere they are separately stored. Based on each of the color imagesignals, a processor/controller calculates a luminous center of gravityfor each color test pattern, calculating a misconvergence by comparisonof respective luminous centers of gravity of the three color testpatterns.

Consequently, the device of the present invention minimizes measurementvariations which is likely to occur due to undesirable movements of thecolor image pickup means relative to the color CRT, ensuring measurementaccuracy at every test.

In the device where a horizontal misconvergence (or verticalmisconvergence) is calculated on a vertical line portion (or ahorizontal line portion) of the test pattern, a vertical sampling area(a horizontal sampling area) is set which has a length which is obtainedby multiplying a pitch between the same vertical color phosphor elements(or the same horizontal color phosphor elements) on the viewing screenand an integer, and respective luminous centers of gravity of the threecolor test patterns is calculated from the three color image signalsfrom the sampling area, and another sampling area is set which isdeviated in a vertical direction (or a horizontal direction) by a givenamount from the former sampling area, and respective luminous centers ofgravity of the three color test patterns is calculated from the threecolor image signals from the another sampling area, and a horizontalmisconvergence is finally calculated from the calculated luminouscenters of gravity of the two sampling areas, the calculation ofluminous centers of gravity of three color test patterns receives aremarkably reduced influence of undesirable movement of the image pickupmeans, such as vertical movement, horizontal movement, or obliquemovement in the course of measurement. Consequently, fluctuation inrepeated measurements is reduced.

These and other objects, features and advantages of the presentinvention will become more apparent upon a reading of the followingdetailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a whole construction of a firstembodiment of the present invention;

FIG. 2 is a diagram showing a construction of a color camera head of thefirst embodiment, including a CCD color image pickup device;

FIG. 3A and FIG. 3B are diagrams showing photographed images of greenphosphor dots when photographing a test pattern with differentmagnifications;

FIG. 4 is a diagram showing a vertical sampling range for calculation ofa horizontal luminous center of gravity of green dot images;

FIG. 5A and FIG. 5B are diagrams showing an arrangement of green, andblue phosphor dots on a horizontal line of a cross hatch pattern tocalculate a vertical luminous center of gravity;

FIG. 6A through FIG. 6D are diagrams showing measurement areas formeasuring convergence over a cross hatch pattern;

FIG. 7 is a flowchart showing a vertical convergence measurement of thefirst embodiment;

FIG. 8 is a flowchart showing another vertical convergence measurementof the first embodiment;

FIG. 9 is a flowchart showing a horizontal convergence measurement ofthe first embodiment;

FIG. 10 is a flowchart showing another horizontal convergencemeasurement of the first embodiment;

FIG. 11A through FIG. 11D are diagrams showing another measurement areasfor measuring convergence over a cross hatch pattern;

FIG. 12 is a diagram showing a construction of a color camera head of asecond embodiment;

FIG. 13 is a diagram showing a magnification sensor of the second colorcamera head, including a differential transformer;

FIG. 14 is a partial schematic diagram of the differential transformer;

FIG. 15 is a graph showing output characteristics of the differentialtransformer, the outputs being plotted as a function of position of alens assembly of the second color camera head;

FIG. 16 is a graph showing a relationship between magnification sensoroutput and position of the lens assembly;

FIG. 17 is a graph showing a relationship between image magnificationand position of the lens assembly;

FIG. 18 is a flowchart showing a control process of a zoom ring foroptimum magnification setting;

FIG. 19 is a diagram of a magnification sensor including apotentionmeter;

FIG. 20 is a diagram showing another magnification sensor including anencoder;

FIG. 21 is a perspective view showing a construction of the encoder;

FIG. 22 is a plan view showing a coded pattern printed circuit board ofthe encoder;

FIG. 23 is a diagram showing a construction of a color camera head of athird embodiment;

FIG. 24 is a flowchart showing a magnification setting process of thethird embodiment;

FIG. 25 is a flowchart showing another magnification setting process ofthe third embodiment;

FIG. 26 is a flowchart showing yet another magnification setting processof the third embodiment;

FIG. 27 is a diagram showing a cross hatch test pattern;

FIG. 28 is an expanded diagram showing an area A shown in FIG. 27;

FIG. 29 is a diagram showing an arrangement of red, green, and bluepixels on a color image pickup device; and

FIG. 30A and FIG. 30B are diagrams showing examples of a green phosphordot image.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a whole construction of a first embodiment of the presentinvention. A composite test pattern for measurement is generated by apattern generator 3, and fed via a driving unit 2 to a color CRT 1 wherethe test pattern is presented on its viewing screen.

The composite test pattern consists of three color test patterns, i.e.,red test pattern, green test pattern, and blue test pattern. A colorcamera head 4, which is placed in front of the CRT while facing theviewing screen, picks up the composite test pattern with a built-in CCDdevice. The color camera head enables an easy setting to requiredmeasurement points one after another over the viewing screen of the CRT1 because the color camera head can be simply pressed manually againstthe viewing screen. The color camera head 4 then separates the picked uptest pattern image into red, green, and blue test pattern images, eachof which is transferred to a video memory 6 provided in a main unit 5for convergence measurement. The main unit 5 comprises the video memory6, a processor/controller 7, a memory 8, a data output circuit 9 and adata input circuit 10. Each of them is described more in detail below.

The video memory 6, made up of a A/D converter and backup memories 6a,6b and 6c, converts analog image data of each color into red, green, andblue digital image data, which are separately stored in backup memories6a, 6b and 6c respectively. With a micro processor, theprocessor/controller 7 controls the operation of the convergencemeasuring device in accordance with a program stored in the memory 8,and also performs arithmetic operations to calculate convergence of theCRT based on the digital data of color image stored in the video memory6. The memory 8 stores the resultant convergence data, which is alsotransferred to a data output circuit 9. A data input circuit 10 receivesexternal data and transfers it to the processor/controller 7.

The color camera head 4 is diagrammed in detail in FIG. 2 to describe afirst embodiment of this invention. With its camera hood 11 directlytouching to the faceplate 1a of the CRT 1, the camera head 4 can be setto a measurement area over the viewing screen. The test pattern withinthe measurement area on the viewing screen is magnified through a lensassembly 12 and then picked up by a color image pickup device 16.

The lens assembly 12 provides a zoom control to control magnification, afocus control and an iris control, controlled by a zooming ring 13, afocussing ring 14 and an iris ring 15 respectively. The zooming ring 13is graduated to indicate phosphor dot pitch numbers. The camera head 4has a fixed ring provided with a reference mark. A manual magnificationsetting is carried out by turning the zooming ring 13 until thereference mark agrees with a graduated pitch number on the zooming ring13 showing the phosphor dot pitch of the color CRT under test.Conversely, the zooming ring 13 may be notched for a reference mark, andthe fixed ring on the camera head may be graduated to indicate pitchnumbers.

The color image pickup device 16 has, in front of the color pixels, abuilt-in color separating filter which has filtering sections arrangedin the same matrix as the color pixels. The pixels pick up the testpattern in the form of a color image. The pixels discriminate thepattern image in accordance with colors at each pixel, and each pixeloutputs signal for one of the three colors. A switch 17 is a measurementstart switch. When it is switched on, a trigger signal is transmitted tothe processor/controller 7.

Described below is how convergence is measured in this embodiment. Aspreviously mentioned, fluctuation in misconvergence measurement changeswith magnification in the image picking up process. As shown in FIGS.30A and 30B, the stripe pattern of the green image caused by picking upone green phosphor dot changes with magnification. Also, respectiveluminous centers of gravity of the green phosphor dots on a line changeswith magnification. Consequently, the green luminous center of gravityof the green phosphor line changes with magnification. Accordingly, itcould be seen that the fluctuation can be reduced by selecting a propermagnification.

Referring to FIG. 3A, the magnification is changed so that the stripepattern of the phosphor dots on the right column and the strip patternof the phosphor dots on the left column come to be identical to eachother. FIG. 3B shows that with the color camera head 4 slightly moved onthe viewing screen of the color CRT 1, the strip pattern of the phosphordots becomes different from that of FIG. 3A. It is noted that thefluctuation of deviation δ between Ms (luminous center of gravity ofphosphor dot itself) and Mm (luminous center of gravity of phosphor dotimage) does not vary regardless of the movement of the color camera head4 on the viewing screen.

From experiments and simulation tests, it has been found that a minimumfluctuation in misconvergence measurement is obtainable with respect tothe vertical movement, horizontal movement, and rotational movement(around the optical axis) of the camera head 4 over the viewing screenof the CRT when the magnification is set at such a value that ahorizontal pitch between the same color phosphor dot images is equal to2n×Pt' (n≧10). Pt' is a pitch between the same color pixels of the imagepickup device. Pt is a vertical pitch of the phosphor dots of one color.Also, it has been found that in the case of setting the color camerahead 4 at the horizontal position or rotating the the pixel arrangementshown in FIG. 29 90 degrees around the optical axis, a minimumfluctuation in misconvergence measurement is obtainable with respect tothe vertical movement, horizontal movement, and rotational movement(around the optical axis) of the camera head 4 over the viewing screenof the CRT when the magnification is set at such a value that a verticalpitch between the same phosphor dot images is equal to (n+1/2)×Pt'(n≧11).

When the pixels of each color of the image pickup device are in a stripearrangement, magnification β of the lens assembly 12 in the camera head4 is set as follows:

    β=20·Pt'/Pe

When the camera head 4 is used with the camera head 4 rotated 90degrees, magnification β is set as follows:

    β=11.5·Pt'/Pt

The above-mentioned equations shows that a constant pitch between dotimages is held in the image pickup device regardless of varying phosphordot pitchs. In the former case, the ratio Po of a phosphor dot pitch toa pixel pitch is 20, and in latter case, Po is 11.5. When the pixelpitch Pt' is 34.5 μm, optimum magnification β is listed with respect toa number of phosphor dot pitches in the following Table.

                  TABLE                                                           ______________________________________                                        Phosphor                                                                      dot pitch    Magnification                                                    Pt(mm)       β                                                           ______________________________________                                        0.48         0.83                                                             0.39         1.02                                                             0.36         1.11                                                             0.31         1.29                                                             0.26         1.53                                                             0.21         1.89                                                             ______________________________________                                    

When the camera head 4 is moved horizontally or vertically, thereference point of measurement moves accordingly. Accordingly,measurements considerably fluctuate with movement of the camera head 4.Such measurement fluctuation, however, can be reduced by setting aproper sampling area for calculation. In measuring the horizontalluminous center of gravity, for example, the sampling area is set whichhas a vertical length equal to the product of the vertical phosphor dotpitch Pt and an integer. The number of phosphor dots within the samplingarea is constant even when the camera head 4 is moved. Accordingly, theluminous center of gravity will be kept constant. FIG. 4 shows twosampling areas which have a vertical length a and a vertical length brespectively. Both the vertical lengths a and b are equal to twice thevertical phosphor dot pitch Pt. It can be seen that the luminous centerof gravity obtained in the sampling area having the vertical length a isidentical to that of the sampling area having the vertical length b.Similarly, the vertical luminous center of gravity can be obtained bysetting a sampling area having a vertical length equal to the product ofthe horizontal phosphor dot pitch Pe and an integer. In the case ofcalculating the vertical luminous center of gravity, however, an erroris likely to occur if the camera head 4 is unintentionally rotated.Accordingly, the following averaging calculation is carried out.

FIG. 5A and FIG. 5B show green and blue phosphor dots in a horizontalline of a cross hatch pattern for calculating the vertical luminouscenter of gravity. Circles drawn by solid lines represent green phoshordots. Circles drawn by broken lines represent blue phosphor dots. InFIG. 5A, a first sampling area is shown to have a horizontal length a'equal to the horizontal phosphor dot pitch ##EQU1## In the firstsampling area are contained two green phosphor dots G1 and G2 and twoblue phosphor dots B1 and B2. The green phosphor dots G1 and G2 make aluminous center Dg of green phosphor dot in the first sampling area.Also, the blue phosphor dots B1 and B2 make a luminous center Db of bluephosphor dot in the first sampling area. Indicated at d is a horizontaldistance between Dg and Db. It should be noted that no verticaldifference occurs between Dg and Db when the camera head 4 is placed inthe horizontal position. However, when the camera head 4 is rotated by θaround its optical axis as shown in FIG. 5A, Db deviates with respect toDg by d×sin θ.

FIG. 5B shows a state in which the first sampling area is moved in ahorizontal direction by a distance l. In the moved sampling area or asecond sampling area are contained two green phosphor dots G1 and G2 andtwo blue phosphor dots B2 and B3. The green phosphor dots G1 and G2 hasthe luminous center Dg. The blue phosphor dots B2 and B3 make a luminouscenter Db' of blue phosphor dot in the second sampling area. Indicatedat d' is a horizontal distance between Dg and Db'. No verticaldifference occurs between Dg and Db' when the camera head 4 is placed inthe horizontal position. However, when the camera head 4 is rotated byθ, Db' deviates with respect to Dg by -d×sin θ. When the distance l isPe/4, the vertical deviation direction of the first sampling area andthat of the second sampling area are opposite to each other. Byaveraging the luminous centers of gravity of the first sampling area andthose of the second sampling area, accordingly, a remarkably reducedmeasurement error is involved compared with measurement errors occuringin only one of the first sampling area and the second sampling area.

Described above is the averaging calculation for the vertical luminouscenters of gravity. Such averaging calculation can be performed in asimilar manner to determine the horizontal luminous centers of gravityby setting two sampling areas with one apart from the other by Pt/4 in avertical direction.

This averaging concept has been described with reference to a color CRThaving a viewing screen provided with phosphor material in the form ofdots. However, it should be noted that this concept is applicable tomeasurement of a color CRT having a viewing screen provided withphosphor material in the form of stripes. In the measurement of thestripe-type color CRT, when the distance l is Pe/2, Pe being a pitchbetween the same color stripes, the vertical deviation direction of afirst sampling area and that of a second sampling area are opposite toeach other. Pe is a pitch between the same color stripes. Accordingly,measurement error is considerably reduced compared with measurements notincluding such averaging calculation.

The following describe how convergence is measured in this embodimentreferring to FIG. 6 through FIG. 11. FIG. 6A illustrates a cross hatchpattern 20 produced on the whole viewing screen on the color CRT 1 to bemeasured. Indicated at 21a through 21i are portions for measurement ofconvergence. The portion 21a is expanded in FIG. 6B. A boxed area 22indicated at FIG. 6B is an image obtained by the image pickup device. Avertical convergence is calculated in a horizontal line 23 of the crosshatch pattern 20, and a horizontal convergence is calculated in avertical line 24 of the cross hatch pattern 20. FIG. 6C shows each ofred, green, and blue horizontal lines separated from a horizontalcomposite line 23 of the portion. In FIG. 6C, a first sampling areahaving a horizontal length x and a second sampling area having ahorizontal length x' are deviated from each other by ##EQU2## to averagemeasurements of the two sampling areas as mentioned above.

FIG. 6D shows each of red, green, and blue lines separated from avertical composite line 24 of the portion. In FIG. 6D, a first samplingarea having a vertical length y and a second sampling area having avertical length y' are deviated from each other by Pt/4 to averagemeasurements of the two sampling areas as mentioned above.

Referring to a flowchart shown in FIG. 7, the measurement process of avertical convergence is described below. Magnification matched to aphosphor dot pitch of the color CRT 1 under test is first manually setby turning the zooming ring 13 on the color camera head 4, and then theset magnification value (or phosphor dot pitch data) is transferred tothe processor/controller 7 via the data input circuit 10 (Step #1). Asalready mentioned, the camera head 4 is set by turning the zooming ring13 in such a way that a desirable magnification corresponds to thephosphor dot pitch of the color CRT under test. Also, the magnificationis used when resultant misconvergence data is converted in to actualdimensional values in μm. At Step #2, the focus of the lens assembly 12is adjusted. Subsequently, the measurement start switch 7 is switched onto operate the color image pickup device 16 to pick up a test pattern onthe viewing screen. The pattern image picked up by the image pickupdevice 16 is separated into red, green, and blue images, each of whichis again separately stored into the video memory 6 in the main unit 5(Step #3). The processor/controller 7 sets a first sampling area havinga horizontal length x (Step #4), and calculates luminous centers ofgravity Rdx of red phosphor dots, Gdx of green phosphor dots, and Bdx ofblue phosphor dots in the first sampling area x based on the color imagedata stored in the video memory 6 (Step #5). The processor/controller 7further calculates vertical differences ΔRx and ΔBx of Rdx and Bdx withrespect to Gdx (Step #6). These are misconvergences in the firstsampling area x. The processor/controller 7 sets a second sampling areahaving a horizontal length x' (Step #7), and calculate luminous centersof gravity Rdx' of red phosphor dots, Gdx' of green phosphor dots, andBdx' of blue phosphor dots in the second sampling area x' based on thecolor image data stored in the video memory 6 (Step #8). Theprocessor/controller 7 further calculates vertical differences ΔRx' andΔBx' of Rdx' and Bdx' with respect to Gdx' (Step #9). These aremisconvergences in the second sampling area x'. The processor/controller7 calculates averaged misconvergences Rx, Bx of the misconvergences ΔRxand ΔBx of the first sampling area x and the misconvergences ΔRx' andΔBx' of the second sampling area x' (Step #10). The averagedmisconvergences Rx and Bx are stored in the memory 8 (Step #11) andsimultaneously transferred to the data output circuit 9 (Step #12).

In the above manner (referred to as Manner 1 hereinafter), it is firstcarried out to calculate misconvergences in each of the first samplingarea x and the second sampling area x', and they are averaged to providea misconvergence of the line. Alternatively, it is first carried out tocalculate luminous centers of gravity in each of the first sampling areax and the second sampling area x', and respective intermediate pointsbetween the luminous centers of gravity of the first sampling area andthe luminous centers of gravity of the second sampling area, which arethen be used to calculate a misconvergence of the line. This manner ishereinafter referred to as Manner 2. Referring to FIG. 8, Manner 2 willbe described below.

Similarly to Manner 1, magnification matched to a phosphor dot pitch ofthe color CRT 1 under test is first manually set by turning the zoomingring 13 on the color camera head 4, and the magnification value (orphosphor dot pitch data) is transferred to the processor/controller 7via the data input circuit 10 (Step #15). In Step #16, the focus of thelens assembly 12 is adjusted. Subsequently, the measurement start switch7 is switched on to operate the color image pickup device 16 to pick upa test pattern on the viewing screen. The pattern image picked up by theimage pickup device 16 is separated into red, green, and blue images,each of which is separately stored in the video memory 6 in the mainunit 5 (Step #17). The processor/controller 7 sets a first sampling areahaving a horizontal length x (Step #18), and calculates luminous centersof gravity Rdx of red phosphor dots, Gdx of green phosphor dots, and Bdxof blue phosphor dots in the first sampling area x based on the colorimage data stored in the video memory 6 (Step #19). Theprocessor/controller 7 sets a second sampling area having a horizontallength x' (Step #20), and calculate luminous centers of gravity Rdx' ofred phosphor dots, Gdx' of green phosphor dots, and Bdx' of bluephosphor dots in the second sampling area x' based on the color imagedata stored in the video memory 6 (Step #21). The processor/controller 7calculates intermediate points Rmx (red), Gmx (green), Bmx (blue)between Rdx and Rdx', Gdx and Gdx', and Bdx and Bdx' respectively (Step#22). The processor/controller 7 further calculates deviations of Rmx,Bmx with respect to Gmx (Step #23). These deviations are misconvergencesRx, Bx of the line. Both Rx and Bx are stored in the memory 8 (Step #24)and simultaneously transferred to the data output circuit 9 (Step #25).

The horizontal convergence measurement in the vertical line 24 shown inFIG. 6B is also carried out in the same manner as in the above mentionedvertical convergence measurement, as flowcharts shown in FIG. 9 and FIG.10. The flowchart in FIG. 9 shows that Manner 1, previously described inconnection with the vertical convergence measurement, is applied to thehorizontal convergence measurement. The flowchart in FIG. 10 shows thatManner 2 is applied to the horizontal convergence measurement.Description of the flowchart steps in FIG. 9 and FIG. 10 are almostidentical to those in FIG. 7 and FIG. 8 respectively except thatsampling areas and calculation are made in a vertical direction.

Flowchart steps for the horizontal convergence measurement will be onlybriefly described below. Referring to the flowchart in FIG. 9, whereManner 1 applies, it is first carried out to calculate misconvergencesΔRy and ΔBy in the first sampling area y with respect to the luminouscenter of gravity of the green phosphor dots (Step #30 through Step#35). It is then carried out to calculate misconvergence ΔRy' and ΔBy'in the second sampling area y' with respect to the luminous center ofgravity of the green phosphor dots (Step #36 through Step #38).Misconvegences Ry and By of the line are obtained by averaging ΔRy andΔRy', and ΔBy and ΔBy' respectively (Step #39). The misconvergences Ryand By are stored in the memory 8 (Step #40), and simultaneouslytransferred to the data output circuit 9 (Step #41).

Referring to the flowchart in FIG. 10, where Manner 2 is applied, it isfirst carried out to calculate luminous centers Rdy (red), Gdy (green)and Bdy (blue) of gravity in the first sampling area y (Step #45 throughStep #49). It is then carried out to calculate luminous centers Rdy'(red), Gdy' (green) and Bdy' (blue) of gravity in the second samplingarea y' (Step #50 and then #51). Subsequently, it is carried out tocalculate intermediate points Rmy (red), Gmy (green), Bmy (blue) betweenRdy and Rdy', Gdy and Gdy', Bdy and Bdy' (Step #53). Furthermore,deviations of Rmy and Bmy with respect to Gmy are calculated (Step #53),which are misconvergences Ry, By of the line in Step 53. Themisconvegences Ry and By are stored in the memory 8 (Step #54), andsimultaneously transferred to the data output circuit 9 (Step #55).

When a color CRT under test is provided with round phosphor dots, thevertical pitch of the phosphor dots is smaller than the horizontal pitchof the phosphor dots. Consequently, the luminous centers of gravity invertical sampling areas are calculated more accurately than the luminouscenters of gravity in horizontal sampling areas. Measurement of ahorizontal convergence receives less influence of rotational movement ofthe camera head 4 than measurement of a vertical convergence.Accordingly, when measuring a horizontal misconvergence, it may becarried out to set only one vertical sampling area as shown in FIG. 11D,which can eliminate the averaging calculation and makes it possible toreduce the measurement time.

Described below is a second embodiment which provides means forcontrolling magnification of the lens assembly 12 to an optimum settingaccording to phosphor dot pitchs of the color CRT 1 in the course ofconvergence measurement.

FIG. 12 is a diagram showing a color camera head of a convergencemeasurement device in conjunction with the second embodiment. Denotednumerals are commonly employed in both FIG. 2 and FIG. 12 when membersare identical. A color camera head 25 provides a zoom control foroptimum setting of magnification of the lens assembly 12, a focuscontrol, and an iris control, controlled by a zooming ring 13, by afocussing ring 14 and by an iris ring 15 respectively. A magnificationsensor 26 detects magnification of the lens assembly 12 measuringpositions of each element in it. The sensor signal produced by themagnification sensor 26 is used to display a phosphor dot pitchcorresponding to the detected magnification of the lens assembly 12 soas to control magnification of the lens assembly 12 to an optimumsetting according to the phosphor dot pitch Pt of the color CRT undertest.

FIG. 13 is a diagram of a magnification sensor 26 which employs theprinciple of a differential transformer. FIG. 14 is a partial schematicdiagram of the differential transformer 27. The differential transformer27 includes a primary coil 28a, a core 29, two secondary coils 28b and28c. The primary coil 28a is electromagnetically coupled with thesecondary coils 28b and 28c via the transformer core 29. The core 29 isconnected to one end of an sensor bar 30. The other of the core 29 isconnected to the lens assembly 12. Accordingly, the core 29 is movedwith the lens assembly 12 along an axis of the coil 28 or in thedirection of Z in FIG. 14. The primary coil 28a is fed with an analogsignal from an oscillator. In response to the movement of the core 29,then the movement of the lens assembly 12, the secondary coils 28b and28c generate an AC voltage proportional to the movement of the core 29.The resulting voltage is fed to an input circuit 31 and then to a signalprocessing circuit 32. The input circuit 31 works as a rectifier and thesignal processing circuit 32 as a converter to DC signals. FIG. 15 showsoutput signal from the signal processing circuit 32 in response to themovement of the core 29. A response curve V1 represents a DC outputvoltage rectified from AC voltage generated by 28b. A response curve V2represents also a DC output voltage rectified from AC voltage generatedby 28c. A response curve Vo represents a DC output voltage rectifiedfrom an AC voltage, Eo, which both secondary coils 28b and 28c yieldwhen configured as shown in FIG. 14 so that output from one coil isreverse to that from the other coil in polarity. Referring to FIG. 15, aresponse curve Vo linearly falls with z reaching a minimum value atPoint M, and then linearly rises again with z. Point M represents zeropoint. As plotted in FIG. 16, the sensor output from the magnificationsensor 26 varies in proportion to positional change of lens members ofthe lens assembly 12. The lens assembly 12, on the other hand, isdesigned in such a way that its magnification factor is proportional tothe position of the lens members of the lens assembly 12 as shown inFIG. 17. Consequently, magnification of the lens assembly 12 is inproportion to the output from the magnification sensor 26. The outputfrom the magnification sensor 26 is fed to the processor/controller 7where it is converted into digital signals with an AD converter beforebeing read by the microprocessor there.

Referring to the flowchart in FIG. 18, convergence measurement processof the color CRT in this embodiment is explained below. In Step #60, themagnification sensor 26 yields an output signal, which directly relatesto positional data of the lens members of the lens assembly 12. Then,the process goes to Step #61 where the output signal is converted into amagnification value. Given the magnification value, the optimum phosphordot pitch P is calculated in Step #62. In Step #63, the calculated pitchP is displayed on the data output 9. The operator compares the pitchvalue P to the phosphor dot pitch Pt of the color CRT under test in Step#64. The operator knows the phosphor dot pitch Pt prior to measuring.When the pitch P and the phosphor dot pitch Pt do not agree with eachother, the magnification of the lens assembly is adjusted by manuallymoving the zooming ring 13 in Step #65 before returning to Step #60. Theoperation of Steps #60 through #64 is repeated until the pitch values Pand the phosphor dot pitch Pt come into agreement (Step #66).

In the first embodiment, as mentioned above, the magnification of thecolor camera head 4 is set by turning the zooming ring 13 until a givenpitch number agrees with a reference mark. In the second embodiment,however, the magnification of the color camera head 4 is set withconfirming pitches corresponding to current magnification of the lensassembly in the process of adjustment by the display unit. Accordingly,more accurate setting is obtainable to reduce measurement fluctuation.

In this embodiment, a potentiometer can serve as an alternative for thedifferential transformer. FIG. 19 is a diagram of the magnificationsensor employing a potentiometer. The magnification sensor 33 functionsas follows: The lens assembly 12 has a sensor bar 34 which is designedto move along the movement of the lens members in the direction Z. Thesensor bar 34 is fitted with a sliding contact Q against thepotentiometer R. The potentiometer R is supplied with reference voltageVe. Pickup voltage Vx appearing at the contact Q changes according tothe movement of the lens members of the lens assembly 12. The outputvoltage Vx from the potentiometer is fed via an operational amplifier 35to a AD converter 36 in which analog signals are converted into digitalsignals before being outputted into the processor/controller 7.

The output signal from the magnification sensor is in proportion to theposition of the lens members of the lens assembly 12 as shown in FIG.16. Also, the magnification of the lens assembly is proportionallydetermined by the position of the lens members of the lens assembly asshown in FIG. 17. Consequently, the magnification value is proportionalto the output from the magnification sensor 33. The magnification sensorusing a potentiometer therefore functions in the same way asmagnification sensor using a differential transformer. The flowchart ofautomatic magnification control, previously described referring to FIG.18, also is applied to the magnification sensor using the potentiometer.

As a second alternative for the differential transformer, an encoderserves in the second embodiment. FIG. 20 is a diagram showing aconstruction of a magnification sensor using an encoder. Themagnification sensor 37 functions as follows: The lens assembly 12 has asensor bar 38 which is designed to move along the movement of the lensmembers in the direction of Z. The sensor bar 38 is fitted with theeight grounded coding brushes 39 as shown in FIG. 21. Each tip of theeight coding brushes remains in contact with one of seven separatelycoded pattern traces (from line 0 through line 7 in FIG. 22) on aprinted circuit board 40. Driven by the lens members in the lensassembly 12, the sensor bar 38 moves in the direction Z.

The eight coding brushes 39 always stay in contact with theircorresponding coded pattern traces on the printed circuit board 40reading their present code which corresponds to the position of the lensmembers of the lens assembly 12. As shown in FIG. 22, the printedcircuit board 40 has line 0 through line 7, each trace consisting ofnumerous small split areas, for example, 256 sections or FF_(H) inhexadecimal number, over the effective area of travel within which thecorresponding brushes 39 can move. Every split section is designed to beconductive or insulated to generate a coding signal. An insulatedsection is hatched as shown in FIG. 22. Each line of 0 through 7 has itsown output line labeled code 0 through code 7 respectively. The eightoutput lines are linked to the digital encoder 41. Based on codedsignals on code 0 through code 7, the digital encoder 41 generates an8-bit coded signal corresponding to the position of the lens members ofthe lens assembly.

When the lens members of the lens assembly stops their motion foradjustment, the eight coding brushes 39 also stop somewhere on the codedpattern traces (from line 0 through line 7). All the coding brushes aregrounded as previously mentioned, any lines of which coded sections areconductive make a short circuit to ground through corresponding brushesin contact with the lines. The other lines of which coded sections areinsulated make an open circuit to ground (thus ungrounded). The digitalencoder 41 generates an 8-bit coded signal (corresponding to theposition of the lens members of the lens assembly) discriminatingbetween the grounded and the ungrounded status at every coded patterntrace and then coding a logical 0 to the grounded circuit and a logical1 to the ungrounded circuit. The magnification sensor therefore outputsdigital positional signals in response to the position of the lensmembers of the lens assembly 12. The digital positional signals areproportional to the position of the lens members of the lens assembly,and furthermore the magnification value is also proportional to theposition of the lens members of the lens assembly as shown in FIG. 17.

The magnification value is proportional to the output from themagnification sensor 37. This magnification sensor functions in the sameway as previously described magnification sensors of a differentialtransformer or a potentiometer. The flowchart already describedreferring to FIG. 18 is also applied to this magnification sensor.

A third embodiment of the present invention will now be described. Thethird embodiment allows the lens assembly 12 to be automaticallyadjusted for proper position setting of its elements comparing a currentmagnification value of the lens assembly 12 to an optimum magnificationsetting. FIG. 23 shows the block diagram of a color camera head of theconvergence measurement device in conjunction with the third embodiment.Denoted numerals are commonly employed in both FIG. 12 and FIG. 23 whenmembers are identical. The magnification sensor 42, as previouslymentioned in the second embodiment, can be one using a differentialtransformer, a potentiometer or an encoder. The lens assembly 12comprises a zoom control, which permits lens members to move for focusadjustment and setting of magnification. The color camera head 41includes a mechanism 43 for moving the lens member of the lens assembly12 for focus adjustment and another mechanism 44 for moving the lensmember for magnification setting. The focus control mechanism 43 iscontrolled by a control data provided by a focus sensing means. Themagnification control mechanism 44 is controlled by a control signalfrom the processor/controller 7.

A convergence measurement process in the third embodiment will bedescribed referring to FIG. 24.

A phosphor dot pitch Pt of the color CRT under test is first inputthrough the data input circuit 10 by the operator (Step #70). Uponreceiving the phosphor dot pitch, the processor/controller 7 willretrieve and execute a magnification determining program stored in thememory 8 to give an optimum magnification β0 suitable to convergencemeasurement (Step #71). Optimum magnification β0, for example, isdetermined according to the following relationship: ##EQU3## as alreadymentioned in the first embodiment.

In Step #72, the processor/controller 7 commands a magnification sensingmeans 42 to sense a magnification value β1 at a current position of thelens members of the lens assembly 12 and read in resulting magnificationvalue β1. Current magnification value β1 is compared to β0 (optimummangnification) in Step #73. If β0 is not equal to β1, it transmits amovement command to the magnification control mechanism 44 for it tomove the lens members of the lens assembly 12 in Step #74. The processreturns to Step #72. If β0 is equal to β1 in Step #73, the processadvances to Step #75 in which convergence measurement is carried out.

Another measurement process applicable in this embodiment will now bedescribed with reference to FIG. 25. In the previous process, a phosphordot pitch of the color CRT is manually entered via the data inputcircuit 10 to the processor/controller 7 to determine the optimummagnification and then to automatically adjust the lens assembly 12 forthe optimum magnification. In the process of FIG. 25, a phosphor dotpitch of the color CRT is firstly calculated from stored image signalsand an optimum magnification is calculated for the calculated phosphordot pitch. Subsequently, the lens assembly 12 is set at the optimummagnification by automatically checking whether a current magnificationof the adjusting lens assembly agrees with the optimum magnification.

The color camera head 41 is placed on the viewing screen of the colorCRT 1a under test so as to pick up a test pattern presented on theviewing screen. The color camera head 41 is set at an arbitrarymagnification β1. The magnification sensing means 42 senses thearbitrary magnification β1 in Step #80. The picked-up image data isstored in the video memory 6 in Step #81. In Step #82, theprocessor/controller 7 calculates a pitch of phosphor dot images whichare magnified by the lens assembly 12 from the stored image data usingFourier transformation, etc., and calculates an actual pitch Pt of thephosphor dots on the viewing screen from the magnification β1 of thelens assembly 12 and the phosphor dot image pitch. Subsequently, theprocessor/controller 7 calculates an optimum magnification β0 suitablefor the actual phosphor dot pitch Pt in Step #83. A currentmagnification β1 of the lens assembly 12 is sensed by the magnificationsensing means 42 in Step #84. The current magnification β1 is comparedto the optimum magnification β0 in Step #85. If the currentmagnification β1 and the optimum magnification β 0 do not agree witheach other, the lens members of the lens assembly 12 is moved by apredetermined amount by the magnification control mechanism 44 inaccordance with a control signal in Step #86, and the process returns toStep #84. If the current magnification β1 and the optimum magnificationβ0 agree with each other, measurement of convergence is carried out inStep #87. The optimum magnification β0 is used to change from amisconvergence calculated from the picked-up image data to an actualdimension of the color CRT.

This process eliminates manual setting of the magnification of the lensassembly 12, and manual inputting of optimum magnification data.Consequently, this process provides easier measurement operation andreduced measurement error.

Furthermore, yet another measurement process applicable in thisembodiment will be described with reference to FIG. 26. In the previoustwo processes, the magnification of the lens assembly 12 isautomatically adjusted by comparing a current magnification to anoptimum magnification. In the process of FIG. 26, the lens assembly 12is automatically adjusted by comparing a pitch of phosphor dot imageswhich are magnified by the lens assembly 12 to a reference value equalto the product of a pixel pitch Pt' of the image pickup device 16 and aspecified integer.

The color camera head 41 is placed on the viewing screen of the colorCRT under test to pick up a test pattern presented on the viewingscreen. The color camera head 41 is set at an arbitrary magnification.The picked-up image data is stored in the video memory 6 in Step #90. InStep #91, the processor/controller 7 calculates a pitch P of phosphordot images which are magnified by the lens assembly 12 from the storedimage data using Fourier transformation, etc. Then, theprocessor/controller 7 compares the pitch P to a reference value in Step#92. The reference value is equal to the product of a pixel pitch Pt' ofthe image pickup device 16 and an integer of 20, or 20Pt', and ispreviously entered in the processor/controller 7. If the pitch P is notequal to 20Pt' in Step #92, the lens members of the lens assembly 12 ismoved by a predetermined amount by the magnification control mechanism44 in accordance with a control signal in Step #93, and the processreturns to Step #90. If the pitch P is equal to 20Pt' in Step #92,measurement of convergence is carried out in Step #94. The magnificationof the lens assembly 12 at the time of P=20Pt', which is detected by themagnification sensing means 42, is used to change from a misconvergencecalculated from the pick-up image data to an actual dimension of thecolor CRT. It should be noted that the factor number to be multiplied bythe pixel pitch Pt' is not limited to 20 and other integers areapplicable.

This process eliminates manual setting of the magnification of the lensassembly 12, and manual inputting of optimum magnification data.Consequently, this process provides easier measurement operation andreduced measurement error.

Furthermore, it would be understood that the foregoing relates to onlythe scope of the present invention as defined by the appended claimsrather than by the description preceding them, and all changes that fallwithin metes and bounds of the claims, or equivalence of such metes andbounds are therefore intended to be embraced by the claims.

What is claimed is:
 1. A device for measuring convergence of a color CRTcomprising:image pickup means for producing three color image signalswith respect to three color test patterns constituting a composite testpattern presented on a viewing screen of a color CRT; determining meansfor determining the image magnification of said image pickup meansaccording to the pitch between color phosphor elements on the viewingscreen, and calculation means for calculating respective luminouscenters of gravity of the three color test patterns from the three colorimage signals, and calculating a misconvergence of the color CRT fromthe calculated luminous centers of gravity and a changed imagemagnification, wherein the composite test pattern is a crosshatchpattern, said calculation means calculates respective vertical luminouscenters of gravity of the three color test patterns on a horizontal lineportion of the cross-hatch pattern to calculate a verticalmisconvergence of the color CRT, and calculates respective horizontalluminous centers of gravity of the three color test patterns on avertical line portion of the cross-hatch pattern to calculate ahorizontal misconvergence of the color CRT.
 2. A device for measuringconvergence of a color CRT comprising:image pickup means for producingthree color image signals with respect to three color test patternsconstituting a composite test pattern presented on a viewing screen of acolor CRT; determining means for determining the image magnification ofsaid image pickup means according to the pitch between color phosphorelements on the viewing screen, including:a lens assembly having amagnification changing lens; drive means for driving said magnificationchanging lens, and means for actuating said drive means so as to obtaina desirable image magnification, and calculating means for calculatingrespective luminous centers of gravity of the three color test patternsfrom the three color image signals, and calculating a misconvergence ofthe color CRT from the calculated luminous centers of gravity and achanged image magnification.
 3. A device according to claim 2 whereinsaid means for actuating including a scale having marks representing anumber of phosphor element pitches, the marks corresponding torespective image magnifications suitable for the number of phosphorelement pitches, whereby the image magnification suitable for a phosphorelement pitch of the CRT is obtainable by actuating said means foractuating so that the mark representing the phosphor element pitchreaches a predetermined position.
 4. A device according to claim 2further comprising:detection means for detecting the image magnificationof said lens assembly; pitch calculation means for calculating aphosphor element pitch suitable for a detected image magnification; anddisplay means for displaying the calculated phosphor element pitchwhereby said actuatable means for actuating is so that the calculatedphosphor element pitch displayed on said display means agrees with thephosphor element pitch of the CRT.
 5. A device for measuring convergenceof a color CRT comprising:image pickup means for producing three colorimage signals with respect to three color test patterns constituting acomposite test pattern presented on a viewing screen of a color CRT;determining means for determining the image magnification of said imagepickup means according to the pitch between color phosphor elements onthe viewing screen, including:a lens assembly having a magnificationchanging lens; magnification change means for changing the imagemagnification of said lens assembly; input means for inputting thephosphor element pitch of the CRT; magnification calculation means forcalculating an image magnification suitable for the input phosphorelement pitch, and control means for controlling said magnificationchange means so that the image magnification of said lens assemblyagrees with the calculated image magnification, and calculation meansfor calculating respective luminous centers of gravity of the threecolor test patterns from the three color image signals, and calculatinga misconvergence of the color CRT from the calculated luminous centersof gravity and a changed image magnification.
 6. A device for measuringconvergence of a color CRT comprising:image pickup means for producingthree color image signals with respect to three color test patternsconstituting a composite test pattern presented on a viewing screen of acolor CRT; change means for changing the image magnification of saidimage pickup means according to the pitch between color phosphorelements on the viewing screen, including:a lens assembly having amagnification changing lens; magnification change means for changing theimage magnification of said lens assembly; pitch detection means fordetecting the phosphor element pitch of the CRT; magnificationcalculation means for calculating an image magnification correspondingto the detected phosphor element pitch, and control means forcontrolling said magnification change means so that the imagemagnification of said lens assembly agrees with the calculated imagemagnification, and calculation means for calculating respective luminouscenters of gravity of the three color test patterns from the three colorimage signals, and calculating a misconvergence of the color CRT fromthe calculated luminous centers of gravity and a changed imagemagnification.
 7. A device for measuring convergence of a color CRTcomprising:image pickup means for producing three color image signalswith respect to three color test patterns constituting a composite testpattern presented on a viewing screen of a color CRT; change means forchanging the image magnification of said image pickup means according tothe pitch between color phosphor elements on the viewing screen,including:a lens assembly having a magnification changing lens;magnification change means for changing the image magnification of saidlens assembly; pitch detection means for detecting a pitch of phosphorelement images; storage means for storing a specified pitch, and controlmeans for controlling said magnification change means so that thedetected pitch of phosphor element images agrees with the storedspecified pitch, and calculation means for calculating respectiveluminous centers of gravity of the three color test patterns from thethree color image signals, and calculating a misconvergence of the colorCRT from the calculated luminous centers of gravity and a changed imagemagnification.
 8. A device for measuring a characteristic of a color CRTcomprising:image pickup means, having color pixels, for producing threecolor images with respect to three color test patterns presented as acomposite test pattern on a viewing screen of the color CRT; means fordetermining the image magnification of the composite test pattern on thecolor pixels according to a pitch between color phosphor elements on theviewing screen and a pitch of the color pixels, and means forcalculating the characteristic value according to the output of theproducing means.
 9. A device according to claim 8 further includingchange means for changing the image magnification so that the pitchbetween color phosphor elements projected on the image pickup meansbecomes a product of an integer number and the pitch between colorpixels, and images of the color pixels under the projected phosphorelements become congruent.
 10. A device as claimed in claim 8 whereinthe calculating means includes means, according to the three colorimages obtained by the focused composite test pattern with thedetermined image magnification, for calculating a convergence valuerepresenting convergence of each electrical beam from cathode ray tubesagainst each color phosphor in the CRT.
 11. A device for measuringconvergence of a color CRT comprising:means, having color pixels, forproducing three color images with respect to three color test patternsconstituting a composite test pattern presented on a viewing screen of acolor CRT; means for determining the image magnification of thecomposite test pattern on the color pixels according to the pitchbetween color phosphor elements on the viewing screen; means forfocusing the image of the composite test pattern on the color pixels,and means for calculating a convergence value according to the threecolor images obtained by the focused composite test pattern with thedetermined image magnification.
 12. A device for measuring acharacteristic of a color CRT which generates a composite test patternincluding three color test patterns on a viewing screencomprising:means, having color pixels, for producing three color imageswith respect to the three color test patterns; means for determining theimage magnification of the composite test pattern on the color pixelsaccording to the pitch between color phosphor elements on the viewingscreen; means for focusing the image of the composite test pattern onthe color pixels, and means for calculating a characteristic valueaccording to the three color images under the in-focus condition by thefocusing means.
 13. A device as claimed in claim 12 wherein thecalculating means includes means, according to the three color images,for calculating a convergence value representing convergence of eachelectrical beam from cathode ray tubes against each color phosphor inthe CRT.